“I don't think I've seen anything like it,” Columbia University seismologist Göran Ekström told National Geographic, before cautioning: “It doesn't mean that, in the end, the cause of them is that exotic.”

Normal earthquakes are short-lived affairs that produce a sudden jolt that typically subsides in seconds, followed by aftershocks which can last far longer. Throughout these standard seismic shifts, three types of waves are produced.

The seismic event shook the world, yet no human being felt the tremors

An earthquake will usually first trigger fast-traveling signals known as Primary or P-waves followed by relatively high-frequency secondary or S-waves.

Thirdly, long-period surface waves travel around the globe multiple times following a big enough earthquake.

While this type of wave most closely resembles the Mayotte mystery signal, there was no earthquake recorded that could have triggered such a bizarre low frequency in the way observed.

“A sort of ping rather than a rumbling,” explains Stephen Hicks, a seismologist at the University of Southampton.

However, the Mayotte event triggered a surprisingly monotonous and low-frequency signal which further mystifies scientists; the wave was monochromatic and took roughly 17 seconds to repeat – completely different to the 'noisy' waves emitted by standard earthquakes.

Most theories point to a seismic swarm that has rocked Mayotte since last May, with hundreds of quakes registered from an area roughly 31 miles offshore, east of the source of the odd ringing.

However, no quakes have been recorded recently and the frequency of seismic activity has subsided in recent months, further confounding scientists investigating this mysterious phenomenon.

The current working theory is that there is a magma body, roughly a cubic mile in size, that is shunting its way through the subsurface of Mayotte and may have caused a subaquatic magma chamber collapse.

Additional ideas suggest that what is known as a 'slow earthquake,' which is quieter than its traditional namesake and can last minutes, hours or even days, is what is behind the enigmatic signal.